Why didn't Ben just take Lindsay on the private jet with him?

Affleck's girlfriend Lindsay Shookus left from LAX back to New York on Friday and the very next day we spotted Ben taking off in a HUGE private jet, solo, headed to the east coast. Why didn't she hitch a ride?

We suspect the couple isn't 100% to commit to going public with their relationship, but they're inching their way toward that. Both have kids to be concerned about and then there's the issue of them having dated for YEARS -- way before Jennifer Garner filed for divorce.

Meanwhile, poor Jen is keeping it together and doing what smart people do -- exercising to relieve stress and, of course, to keep her bod looking tip top. You go, girl!